What kind of screen? How textured is it? And is it more white or the silver reflective stuff?

I'd start with just warm water and a sponge. upgrade to dish soap if that doesn't work. Wipe with a sponge and have towel underneath to catch the water dripping down the screen.

Tell your kid that pizza thrower is not allowed back at your house, and that he can't hang out with him outside of school. keep bad influences away early and often.